Variable sensitivity imaging device including a pulse voltage applying section, and imaging apparatus including the same

ABSTRACT

A variable sensitivity imaging device comprises: a substrate; a photosensitive layer which is stacked above the substrate, and which is interposed between a pixel electrode layer and an opposing electrode layer; a signal reading section, formed on the substrate, that reads a signal corresponding to photo-charges which are generated by incidence of light into the photosensitive layer; and a pulse voltage applying section that applies a variable pulse-width pulse voltage between the pixel electrode layer and the opposing electrode layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an imaging device and an imagingapparatus, and more particularly to a variable sensitivity imagingdevice in which the sensitivity is variable, and an imaging apparatusincluding it.

2. Description of the Related Art

In the related art, several attempts to optimize the sensitivity of animaging device have been made. In the related-art technique disclosed inJP-A-5-111037, color filters which are disposed on a light receivingportion of a solid-state imaging device are driven by an actuator toenable the color filters to be selected. However, the reference does notconcern itself with the sensitivity of the imaging device.

In the related-art technique disclosed in JP-A-5-244609, a material inwhich the spectral transmittance is changed by an applied voltage isused as a color filter, and the sensitivity can be variably set for eachpixel. The sensitivity difference among colors due to the colordifference of the color filter can be reduced. However, the techniquedoes not optimize the sensitivity of an imaging device.

In the related-art technique disclosed in JP-A-9-148549, an on-chip lensis formed by a color filter, and, depending on the sensitivity of apixel for each color, the height of the on-chip lens for the pixel isvaried, thereby correcting the sensitivity difference among colors. Inthe related-art technique disclosed in JP-A-9-163383, a similar effectis attained by adjusting the output gain of an imaging device.

In these imaging devices, strictly speaking, the sensitivity of animaging device itself is not changed, but the sensitivity is adjusted bythe color filter or the on-chip microlens disposed on the front face ofthe light receiving portion of the imaging device, or an outputamplifier. In order to change the sensitivity, therefore, an additionalprocess is required, and there arises a problem on that the productioncost is increased.

Furthermore, the technique in which the sensitivity is made variable bythe microlens has a problem in that there is no real time property.

The imaging devices of the above-described related-art techniques have aconfiguration in which a photodiode is formed in a surface portion of asemiconductor substrate, and photo-charges that are accumulated in thephotodiode by incidence of light are read out to the outside of theimaging device by a signal reading section (a charge transfer path inthe case of the CCD type, or a MOS transistor circuit in the case of theCMOS type).

As disclosed in JP-A-58-103165, JP-A-1-300575 and JP-A-2003-332551,however, there are imaging devices having a configuration in whichphotoelectric converting layers such as organic layers are stacked abovea semiconductor substrate, and photo-charges are generated in accordancewith the amount of incident light, and then read out to the outside ofthe imaging device. Also in such organic imaging devices, however, thereis no device in which the sensitivity of the imaging device itself ischanged.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a variable sensitivityimaging device in which photoelectric converting layers such as organiclayers are stacked above a semiconductor substrate, and the sensitivitycan be adjusted by optimally adjusting the amount of photo-charges thatare generated in the photoelectric converting layers in accordance withthe amount of incident light. It is another object of the invention toprovide an imaging apparatus which is equipped with the variablesensitivity imaging device.

The variable sensitivity imaging device of the invention comprises: asubstrate; a photosensitive layer which is stacked above the substrate,and which is interposed between a pixel electrode layer and an opposingelectrode layer; a signal reading section, formed on the substrate, thatreads a signal corresponding to photo-charges which are generated byincidence of light into the photosensitive layer; and a pulse voltageapplying section that applies a variable pulse-width pulse voltagebetween the pixel electrode layer and the opposing electrode layer.

According to the invention, there is provided the variable sensitivityimaging device, wherein a plurality sets each comprising thephotosensitive layer, and the pixel electrode layer and the opposingelectrode layer between which the photosensitive layer is interposed arestacked, and photosensitive layers of the sets have a peak of a photosensitivity at different wavelength regions.

According to the invention, there is provided the variable sensitivityimaging device, wherein three sets each comprising the photosensitivelayer, and the pixel electrode layer and the opposing electrode layerbetween which the photosensitive layer is interposed are stacked, andfirst one of the three sets has a photo sensitivity at red light, secondone of the three sets has a photo sensitivity at green light, and thirdone of the three sets has a photo sensitivity at blue light.

According to the invention, there is provided the variable sensitivityimaging device, wherein the substrate is a semiconductor substrate, andthe signal reading section comprises: a device having a chargetransferring portion that transfers the photo-charges of a pixel at apredetermined position; or a device having a reading mechanism thatselectively reads a signal corresponding to the photo-charges of thepixel at the predetermined position.

According to the invention, there is provided the variable sensitivityimaging device, wherein the substrate is flexible.

The imaging apparatus of the invention comprises: the above-mentionedvariable sensitivity imaging device; and an applied voltage adjustingsection that controls a time width of the pulse voltage to be appliedbetween the pixel electrode layer and the opposing electrode layer bythe pulse voltage applying section, to adjust an image signal which isoutput from the variable sensitivity imaging device.

The imaging apparatus of the invention comprises: a variable sensitivityimaging device including three sets each comprising the photosensitivelayer, and the pixel electrode layer and the opposing electrode layerbetween which said photosensitive layer is interposed are stacked; andan applied voltage adjusting section that controls a time width of thepulse voltage to be applied between the pixel electrode layer and theopposing electrode layer in each of the three sets by the pulse voltageapplying section, to adjust image signals of red, green, and blue whichare to be output from the variable sensitivity imaging device, andperform white balance adjustment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a digital camera in anembodiment of the invention;

FIG. 2 is a partial surface diagram of a variable sensitivity imagingdevice of the embodiment of the invention;

FIG. 3 is a section diagram taken along the line III-III in FIG. 2;

FIG. 4 is a diagram illustrating an imaging device of a photosensitivelayer (photoelectric converting film) stack type;

FIG. 5 is a view showing the structural formula of quinacridone which isan example of the material used as a photosensitive layer;

FIG. 6 is a graph showing the spectral characteristic of quinacridone;

FIG. 7 is a diagram illustrating a pulse width control of a pulsevoltage which is used in a CCD variable sensitivity imaging device;

FIG. 8 is a diagram illustrating a pulse width control (duty control) ofa pulse voltage which is used in a CMOS variable sensitivity imagingdevice;

FIG. 9 is a flowchart showing the procedure of an auto white balanceprocess which is executed by the digital camera shown in FIG. 1; and

FIG. 10 is a functional block diagram of the related-art digital camera.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an embodiment of the invention will be described withreference to the accompanying drawings.

FIG. 1 is a functional block diagram of a digital camera which is anexample of an imaging apparatus of an embodiment of the invention. Thedigital camera of the embodiment which takes a motion picture or a stillpicture comprises a lens 2, an aperture/shutter 3, and an opticallow-pass filter 4 on the light incident side of a variable sensitivityimaging device 1, and further comprises: an analog signal processingcircuit 5 which receives image signals of R (red), G (green), and B(blue) constituting imaging data of the variable sensitivity imagingdevice 1, and which performs an analog process such as a correlationdual sampling process, an A/D conversion process, or a color separationprocess; a digital signal processing circuit 6 which receives digitalimage signals that have undergone the analog process, and which performsa signal process such as an RGB/YC conversion process to producephotographed image data; a monitor 7 which displays the photographedimage data that have undergone the digital process; an compressionsignal processing circuit 8 which compresses the photographed image datathat have undergone the digital process, to photographed image data ofthe JPEG format or the like; and a memory card 9 which records thephotographed image data that have been compressed.

The digital camera further comprises: an AWB integrating circuit 10which receives the R, G, and B image signals that are output from theanalog signal processing circuit 5, and which integrates the colorsignals in order to perform an auto white balance (AWB) process; an AEintegrating circuit 11 which receives the R, G, and B image signals thatare output from the analog signal processing circuit 5, which integratesthe color signals in order to perform an automatic exposure (AE)process, which outputs a results of the integration to the digitalsignal processing circuit 6, and which controls the aperture/shutter 3in accordance with the integration result; and an AF integrating circuit12 which receives the R, G, and B image signals that are output from theanalog signal processing circuit 5, which integrates the color signalsin order to perform an automatic focusing (AF) process, and whichadjusts the focus position of the lens 2.

The digital camera of the embodiment further comprises a timinggenerator 13, and an applied voltage adjusting circuit 14. In accordancewith a result of the integration of the AWB integrating circuit 10, theapplied voltage adjusting circuit 14 adjusts the time width (pulsewidth) of a pulse voltage to be applied between a pixel electrode layerand an opposing electrode layer (which will be described later) of R andB pixels of the variable sensitivity imaging device 1, thereby adjustingthe sensitivity of the variable sensitivity imaging device 1 itself. Thetiming generator 13 receives the result of the integration of the AEintegrating circuit 11, and outputs a timing of driving the variablesensitivity imaging device 1 to the variable sensitivity imaging device1, at a timing when it cooperates with the applied voltage adjustingcircuit 14.

FIG. 10 is a functional block diagram of the related-art digital camerawhich is illustrated for comparison. The digital camera comprises anR-signal amplifier 21, a G-signal amplifier 22, and a B-signal amplifier23 in a stage subsequent to an analog signal processing circuit. Theamplification gains of the amplifiers 21, 23 are controlled inaccordance with the integration result of the AWB integrating circuit,to attain the auto white balance. By contrast, the digital camera of theembodiment is configured so that, as described with reference to FIG. 1,the time width of the pulse voltage to be applied to the variablesensitivity imaging device 1 itself is controlled in accordance with theintegration result of the AWB integrating circuit 10, to attain the autowhite balance.

FIG. 2 is a partial surface diagram of the variable sensitivity imagingdevice 1 of the embodiment. In the illustrated example, many pixels 25are arranged in a square lattice pattern in the surface of the variablesensitivity imaging device 1. In a bottom portion of each of the pixels25, signal read circuits 26 which read out the R, G, and B image signalscorresponding to signal charges of R (red), G (green), and B (blue)detected by the pixel 25 are formed.

In the embodiment, signal read circuits of the three-transistorconfiguration which is used in a CMOS image sensor are illustrated asthe signal read circuits 26. Alternatively, signal read circuits of thefour-transistor configuration may be used. For each pixel, three signalread circuits 26 are disposed. When designated by a vertical shiftregister 27 and a horizontal shift register 28, the signal read circuitsread out detection signals of R, G, and B to the analog signalprocessing circuit 5 (FIG. 1).

FIG. 3A is a section diagram taken along the line III-III in FIG. 2, andcorresponds to a section of about 1.5 pixels. A diode portion 31 whichis a signal charge accumulating region for red (R) is formed in apredetermined place of a surface portion of a semiconductor substrate30, a diode portion (32) which is a signal charge accumulating regionfor green (G) is formed in a place inner than the diode portion 31 inthe plane of the sheet, and a diode portion (33) which is a signalcharge accumulating region for blue (B) is formed in a further innerplace in the plane of the sheet.

An n-region 34 which constitutes a part of the transistors of the signalread circuits 26 is formed in the surface portion of the semiconductorsubstrate 30. When a read voltage is applied to a gate electrode 35which is disposed via a surface oxide film (not shown) of thesemiconductor substrate 30, accumulated charges in the diode portion 31are moved to the n-region 34, and then read out to the outside of thevariable sensitivity imaging device 1 by the signal read circuit.

As the signal read circuits 26, an R-signal reading circuit which readsout accumulated charges in the diode portion 31, a G-signal readingcircuit which reads out accumulated charges in the diode portion 32, anda B-signal reading circuit which reads out accumulated charges in thediode portion 33 are disposed for each pixel. These signal read circuits26 are light shielded by a light shielding film 38 which is embedded inan insulating layer 37 stacked on the surface of the semiconductorsubstrate 30. In the insulating layer 37, lines 40 through which thesignal read circuits are connected to the vertical shift register 27 andthe horizontal shift register 28 of FIG. 2 are laid above the lightshielding film 38.

An R-pixel electrode layer 41 is stacked on the surface of theinsulating layer 37, and a longitudinal line 42 through which theR-pixel electrode layer 41 is connected to the diode portion 31 islongitudinally formed. A photosensitive layer (photoelectric convertinglayer) 43 having a sensitivity at red light is stacked on the R-pixelelectrode layer 41, and a transparent opposing electrode layer 44 isstacked on the photosensitive layer.

A transparent insulating layer 45 is stacked on the opposing electrodelayer 44, and a transparent G-pixel electrode layer 46 is stacked on theinsulating layer. A longitudinal line (a portion inner than thelongitudinal line 42 in the plane of the sheet: see the referencenumeral 42 g in the right side of FIG. 3B) which is not shown, andthrough which the G-pixel electrode layer 46 is connected to the G-diodeportion 32 is longitudinally formed. A photosensitive layer(photoelectric converting layer) 47 having a sensitivity at green lightis stacked on the G-pixel electrode layer 46, and a transparent opposingelectrode layer 48 is stacked on the photosensitive layer.

A transparent insulating layer 49 is stacked on the opposing electrodelayer 48, and a transparent B-pixel electrode layer 50 is stacked on theinsulating layer. A longitudinal line (a portion which is further innerthan the longitudinal line 42 in the plane of the sheet) which is notshown, and through which the B-pixel electrode layer 50 is connected tothe B-diode portion 33 is longitudinally formed. A photosensitive layer(photoelectric converting layer) 51 having a sensitivity at blue lightis stacked on the B-pixel electrode layer 50, and a transparent opposingelectrode layer 52 is stacked on the photosensitive layer.

The R-longitudinal line 42, the G-longitudinal line, and theB-longitudinal line connect the corresponding diode portions to thepixel electrode layers, respectively, and are electrically insulatedfrom the other members. A transparent protective film (not shown) isstacked on the opposing electrode layer 52.

When light from an object is incident on the thus configured variablesensitivity imaging device 1, light in the wavelength region of blue inthe incident light is absorbed by the B-photosensitive layer 51,hole-electron pairs corresponding to the amount of the absorbed lightare generated, and the electrons in the pairs are flown into the B-diodeportion 33 from the B-pixel electrode layer 50 through the longitudinalline to be accumulated therein.

Similarly, light in the wavelength region of green in the incident lightis absorbed by the G-photosensitive layer 47, hole-electron pairscorresponding to the amount of the absorbed light are generated, and theelectrons in the pairs are flown into the G-diode portion 32 from theG-pixel electrode layer 46 through the longitudinal line to beaccumulated therein.

Similarly, light in the wavelength region of red in the incident lightis absorbed by the R-photosensitive layer 43, hole-electron pairscorresponding to the amount of the absorbed light are generated, and theelectrons in the pairs are flown into the diode portion 31 from theR-pixel electrode layer 41 through the longitudinal line to beaccumulated therein.

Hole-electron pairs which are generated in a photosensitive layer as aresult of light incidence sometimes recombine with each other in thephotosensitive layer. In the imaging device of the embodiment,therefore, a line serving as a section for applying a pulse voltagebetween a pixel electrode layer and an opposing electrode layer isdisposed (this line may be disposed in any manner as far as a desiredcontrol voltage can be applied between the pixel electrode layer and theopposing electrode layer), and, through the line, the applied voltageadjusting circuit 14 shown in FIG. 1 applies a pulse voltage which iscontrolled to have a desired pulse width, and controls a time width atwhich a potential gradient is produced in the photosensitive layerbetween the pixel electrode layer and the opposing electrode layer.

According to the configuration, the time length in which ionizedelectrons of hole-electron pairs are rapidly moved to the pixelelectrode layer and holes to the opposing electrode layer is controlled,recombination of hole-electron pairs is suppressed, and the sensitivityof the imaging device can be adjusted.

FIG. 4 is a section diagram of a case where only one photosensitivelayer is stacked on an opaque substrate. In the diagram of FIG. 3, thestructure in which each photosensitive layer is interposed between apixel electrode layer and an opposing electrode layer is illustrated.Preferably, each photosensitive layer is configured as illustrated inFIG. 4.

A hole blocking layer 56 is formed by Alq on a pixel electrode layer 55made by thin aluminum. On the layer, a photoelectric converting materialis stacked to be formed as a photosensitive layer 57, and a transparentopposing electrode layer 58 is formed by ITO or Au on the photosensitivelayer.

Each of films of aluminum, the photoelectric converting material, andAlq can be formed by vacuum evaporation. Preferably, the degree ofvacuum is about 10⁻⁴ Pa. When a voltage is applied between a pixelelectrode layer and an opposing electrode layer, a large dark currentdue to, particularly, injection of holes flows. Therefore, Alq isrequired as a hole blocking layer.

While preventing injection of holes from the electrode 55 fromoccurring, the hole blocking layer functions to receive electroncarriers generated in the photosensitive layer (photoelectric convertinglayer) 57, and transport the electron carriers to the electrode 55.Although small, the hole blocking layer has a sensitivity.

The opposing electrode layer (ITO, Au, or the like) 58 can be formed bysputtering, electron beam evaporation, ion plating, or the like. In thecase where an organic layer is used as the photosensitive layer 57, whenthe ITO 58 is formed on the organic layer 57, usually, the yield is veryimpaired by a short circuit. When the thickness is less than about 10nm, the yield is improved.

In the case where the organic layer is largely damaged, preferably, athin film of gold (Au) is used as the opposing electrode layer 58although the light transmittance is lower as compared with ITO. Also inthis case, preferably, the thickness is less than about 15 nm.

When the photosensitive layer 57 has a thickness of about 100 nm, alsoreflection from the aluminum electrode layer 55 exists, and 90 to 99% ofincident light can be absorbed. The voltage applied between the pixelelectrode layer 55 and the opposing electrode layer 58 is usually about1 to 30 V. At about 15 V, the external quantum efficiency at the maximumabsorbance wavelength is about 20 to 40%. When the voltage is furtherraised, the quantum efficiency is enhanced, but the dark current due toinjection of holes from the electrode 55 is increased, so that the S/Nis impaired.

The photosensitive layer 57 made of an organic material is deterioratedby oxygen or water. Therefore, a sealing layer of silicon nitride or thelike is necessary on the opposing electrode layer 58 (in FIG. 3, theopposing electrode layer 52). In this case, the sealing layer may beformed by low-damage sputter, low-damage plasma CVD, or the like so asnot to damage the device.

Examples of the material of the photosensitive layer 57 are copperphthalocyanine, porphyrin, Me-PTC, and quinacridone. The characteristicof quinacridone will be described as one example. FIG. 5 shows thestructural formula of quinacridone, and FIG. 6 is a graph showing thespectral sensitivity characteristic of quinacridone.

According to FIG. 6, quinacridone has a spectral characteristic which issimilar to the human visual sensitivity characteristic, and hence can beused as a monochrome photosensitive layer, or as a photosensitive layerfor green (G). Quinacridone has a sensitivity peak also in the shortwavelength side. The lens 2 (see FIG. 1, more correctly glass used inthe lens) has a function of absorbing short-wavelength light, and hencecoating is applied as required to the lens 2 and the like. Therefore,the peak of the short wavelength side can be easily eliminated, andhence there arises no problem.

FIGS. 7 and 8 are diagrams illustrating the pulse voltage which isapplied by the applied voltage adjusting circuit 14 shown in FIG. 1between the pixel electrode layer and the opposing electrode layer. Inthe variable sensitivity imaging device which has been described withreference to FIGS. 2 and 3, the signal reading section formed in thesurface portion of the semiconductor substrate 30 is configured by thesignal read circuits of the transistor configuration which is identicalwith that of as the related-art CMOS image sensor. Alternatively, thesignal reading section may be configured by charge transfer paths (avertical transfer path (VCCD) and a horizontal transfer path (HCCD))formed by the same register configuration as the related-art CCD imagesensor. A method of controlling the width of the pulse to be appliedbetween the pixel electrode layer and the opposing electrode layer isdifferent depending on whether the device is a variable sensitivityimaging device in which the signal reading section is configured by atransistor circuit, or a variable sensitivity imaging device in whichthe signal reading section is configured by charge transfer paths.

FIG. 7 is a diagram illustrating the pulse width control in a variablesensitivity imaging device in which the signal reading section isconfigured by charge transfer paths. In the CCD variable sensitivityimaging device, exposure is started after application of a drawout pulse(not shown) for the electronic shutter is ended. The exposure time shownin the figure indicates a time period from the end of the application ofthe electronic shutter pulse to application of a transfer pulse fortransferring charges of pixels to the vertical transfer path (VCCD).

During the exposure time (a time period when the electronic shutter isopened), light is incident on the photosensitive layer, andhole-electron pairs are generated. In the time period when thehole-electron pairs are generated in the photosensitive layer, onlyduring a time period when the pulse voltage is applied between the pixelelectrode layer and the opposing electrode layer, a potential gradientis produced in the photosensitive layer. Because of the potentialgradient, holes are rapidly moved toward the opposing electrode layer,and electrons toward the pixel electrode layer, with the result thatelectrons are flown into the signal charge accumulating portions (thediode portions 31, 32, 33).

FIG. 8 is a diagram illustrating the pulse width control in a CMOSvariable sensitivity imaging device in which the signal reading sectionis configured by CMOS transistors. In the case of the CMOS type, eventhe pulse width control is performed in the same manner as the CCD type,the sensitivity is not well controlled.

In the case of the CMOS type, the electronic shutter is driven by therolling shutter method. The pixels are sequentially reset in such amanner that the resetting operation is advanced with starting from theupper left pixel of the screen and in the rightward direction, and, inthe next row, from the left pixel and in the rightward direction. Thetime period to the next application of the read pulse is the exposuretime. Unlike in the CCD type, the pixels are not simultaneously exposed.When the pulse width control of FIG. 7 is performed, therefore, theupper and lower portions of the screen are different in exposure time.

In the CMOS type, therefore, the duty control of the pulse voltage isperformed during the time period when the electronic shutter is opened,as shown in FIG. 8. Only a desired time period in the time period whenthe electronic shutter is opened, the time when a potential gradient isproduced in the photosensitive layer is adjusted, or namely thesensitivity is adjusted.

FIG. 8 shows a case where the time when a potential gradient is producedin the photosensitive layer is controlled to be one half of the timeperiod when the electronic shutter is opened. As a result, also thesensitivity is controlled to be one half. In FIG. 8, a potentialgradient is produced by applying four pulses each having a width whichis one eighth of the exposure time. As the pulse width is finer (even inthe unit of a clock signal), a photographed image is smoother (when animage is coarse, the image is seen as if several images overlap witheach other).

FIG. 9 is a flowchart showing the procedure of the auto white balance(AWB) process which is executed by the digital camera of the embodiment.When the AWB process is started, the AWB integrating circuit 10 firstintegrates the R, G, and B image signals to obtain a result of theintegration (step S1). Next, it is determined whether the integrationvalue of R is equal to that of G in a predetermined range or not (stepS2). If the result relating to the range is affirmative, the controlproceeds to next step S3, and it is determined whether the integrationvalue of B is equal to that of G in a predetermined range or not. If theresult of the decision is affirmative, the AWB process is ended.

In the case where a color image is photographed, usually, it is assumedin any scene that the image contains the same amounts of red (R), green(G), and blue (B). If the results of steps S2 and S3 are affirmative(R≅G≅B), therefore, it can be determined that a color image whichsatisfies the assumption is photographed. In the AWB process, theadjustment control by the applied voltage adjusting circuit 14 is notperformed.

If the result of the decision of step S2 is negative, the controlproceeds from step S2 to step S4, and it is determined whether theintegration value of R>the integration value of G is established or not.If established, it is determined that the sensitivity for red isexcessively high, and the control proceeds to step S5. The applicationtime (pulse width) of the pulse voltage to be applied between the pixelelectrode layer 41 and the opposing electrode layer 44 of theR-photosensitive layer 43 is decreased. Then, the control returns tostep S1. Namely, a feedback control is performed, and the sensitivity isadjusted in real time (this is applicable in the following description).

If the result of the decision of step S4 is negative, or the integrationvalue of red in the color image is considerably smaller than that ofgreen, it is determined that the sensitivity for red is excessively low,and the control proceeds to step S6. The application time of the pulsevoltage to be applied between the pixel electrode layer 41 and theopposing electrode layer 44 of the R-photosensitive layer 43 isincreased. Then, the control returns to step S1.

If the result of the decision of step S3 is negative, the controlproceeds from step S3 to step S7, and it is determined whether theintegration value of B>the integration value of G is established or not.If established, it is determined that the sensitivity for blue isexcessively high, and the control proceeds to step S8. The applicationtime of the pulse voltage to be applied between the pixel electrodelayer 50 and the opposing electrode layer 52 of the B-photosensitivelayer 51 is decreased. Then, the control returns to step S1.

If the result of the decision of step S7 is negative, or the integrationvalue of blue in the color image is considerably smaller than that ofgreen, it is determined that the sensitivity for blue is excessivelylow, and the control proceeds to step S9. The application time (pulsewidth) of the pulse voltage to be applied between the pixel electrodelayer 50 and the opposing electrode layer 52 of the B-photosensitivelayer 51 is increased. Then, the control returns to step S1.

As a result, in the digital camera which is equipped with the variablesensitivity imaging device 1 of the embodiment, the sensitivities of thered, green, and blue pixels (partial pixels for the colors in one pixel25 of FIG. 2) can be controlled so as to be substantially uniform, andit is possible to photograph an excellent color image.

When a dark scene in which the whole amount of incident light is smallis to be photographed, an operation switch (not shown) of the digitalcamera is operated so as to increase the ISO sensitivity. As a result,the application times of pulse voltages to be applied between the pixelelectrode layer and the opposing electrode layer in the pixels areincreased as a whole, and the rate of extinction due to recombination ofhole-electron pairs generated in each photosensitive layer is reduced,so that it is possible to photograph a bright and low-noise image.

In the variable sensitivity imaging device of the above-describedembodiment, the signal read circuits of the three- or four-transistorconfiguration which is employed in the related-art CMOS image sensor areused as the signal reading section. Alternatively, a charge transferpath configured by a register which is employed in the related-art CCDimage sensor may be used as the signal reading section. The signalcharge accumulating portions are configured by the diodes.Alternatively, the signal charge accumulating portions may be configuredby capacitors.

In the digital camera in the above-described embodiment, the shutter 3is disposed. The shutter is used in order merely to decide the temporalsimultaneity of a photographed image. Depending on the configuration ofa variable sensitivity imaging device, a shutter is not alwaysnecessary. For example, a shutter is not essential when a CMOS variablesensitivity imaging device having a mechanism which is called a globalshutter, and in which photo-charges can be temporarily accumulated isconfigured, or when a CCD variable sensitivity imaging device in whichsimultaneous reading is enabled is configured.

In the above-described embodiment, the variable sensitivity imagingdevice has a configuration in which the photosensitive layers arestacked above the semiconductor substrate. Alternatively, the signalreading circuits may be formed by using a technique in which, in placeof a semiconductor substrate, a flexible sheet formed by a material suchas PET (polyethylene terephtalate) is used as a substrate, and, forexample, a TFT matrix of a liquid crystal substrate such as disclosed inJP-A-5-158070 is produced on the surface portion of the substrate, or atechnique in which organic EL devices are produced on such a substrate.

According to the invention, the sensitivity of the imaging device can bevariably controlled by means of the pulse width, and photographing of anexcellent image is enabled. Furthermore, the sensitivity can beoptimally adjusted for each color. Therefore, photographing of a colorimage in which the white balance is excellent is enabled.

According to the variable sensitivity imaging device of the invention,the sensitivity of the imaging device itself can be variably controlled,and hence photographing of an excellent image is enabled. Therefore, thevariable sensitivity imaging device is useful as an imaging device to bemounted on a digital camera which takes a motion picture or a stillpicture, such as a digital video camera or a digital still camera.

The entire disclosure of each and every foreign patent application fromwhich the benefit of foreign priority has been claimed in the presentapplication is incorporated herein by reference, as if fully set forth.

1. A variable sensitivity imaging device comprising: a substrate; afirst photosensitive structure comprising: a first pixel electrodelayer; a first opposing electrode layer; and a first photosensitivelayer that is stacked above the substrate, and that is interposedbetween the first pixel electrode layer and the first opposing electrodelayer; a second photosensitive structure comprising: a second pixelelectrode layer; a second opposing electrode layer; and a secondphotosensitive layer that is stacked above the substrate, and that isinterposed between the second pixel electrode layer and the secondopposing electrode layer; a signal reading section, formed on thesubstrate, that reads a first signal corresponding to photo-charges thatare generated in the first photosensitive structure by incidence oflight and a second signal corresponding to photo-charges that aregenerated in the second photosensitive structure by the incidence oflight; and a pulse voltage applying section that applies a first pulsevoltage between the first pixel electrode layer and the first opposingelectrode of the first photosensitive structure, and a second pulsevoltage between the second pixel electrode layer and the second opposingelectrode of the second photosensitive structure, wherein the firstphotosensitive layer of the first photosensitive structure is stackedabove the second photosensitive layer of the second photosensitivestructure, and wherein a time width of the first pulse voltage isdifferent from a time width of the second pulse voltage.
 2. A variablesensitivity imaging device according to claim 1, wherein the firstphotosensitive layer of the first photosensitive structure has a peak ofa photo sensitivity in wavelength from that of the second photosensitivestructure.
 3. A variable sensitivity imaging device according to claim2, further comprising: a third photosensitive structure comprising: athird pixel electrode layer; a third opposing electrode layer; and athird photosensitive layer that is stacked above the substrate, and thatis interposed between the third pixel electrode layer and the thirdopposing electrode layer, wherein a first one the first photosensitivestructure, the second photosensitive structure, and the thirdphotosensitive structure has a photo sensitivity at red light, wherein asecond one of the first photosensitive structure, the secondphotosensitive structure, and the third photosensitive structure has aphoto sensitivity at green light, and wherein a third one of the firstphotosensitive structure, the second photosensitive structure, and thethird photosensitive structure has a photo sensitivity at blue light. 4.A variable sensitivity imaging device according to claim 1, wherein thesubstrate comprises a semiconductor substrate, and wherein the signalreading section comprises: a device having a charge transferring portionthat transfers the photo-charges of a pixel at a predetermined position;or a device having a reading mechanism that selectively reads a signalcorresponding to the photo-charges of the pixel at the predeterminedposition.
 5. A variable sensitivity imaging device according to claim 1,wherein the substrate is flexible.
 6. An imaging apparatus comprising:the variable sensitivity imaging device according to claim 1; and anapplied voltage adjusting section that controls the time width for thefirst photosensitive structure, the second photosensitive structure, andthe third photosensitive structure, to adjust an image signal outputfrom the variable sensitivity imaging device.
 7. An imaging apparatuscomprising: the variable sensitivity imaging device according to claim3; and an applied voltage adjusting section that controls the time widthfor the first voltage, the second voltage, and a third voltage, toadjust image signals of red, green, and blue that are to be output fromthe variable sensitivity imaging device, and perform white balanceadjustment.